The catalog pg_class catalogs
tables and most everything else that has columns or is otherwise
similar to a table. This includes indexes (but see also
pg_index), sequences, views,
composite types, and TOAST tables; see relkind. Below, when we mean all of these
kinds of objects we speak of "relations". Not all columns are meaningful for
all relation types.

All transaction IDs before this one have been
replaced with a permanent ("frozen") transaction ID in this table.
This is used to track whether the table needs to be
vacuumed in order to prevent transaction ID wraparound or
to allow pg_clog to be shrunk.
Zero (InvalidTransactionId) if
the relation is not a table